1. Field of the Invention
The invention relates to measuring and testing generally and to fluid pressure measurement, in particular.
2. Description of the Related Art
There are several methods for the measurement of pressures in fluid dynamics research. One particularly useful approach employs either piezoresistive or metallic strain gauges bonded to either a silicon or a stainless steel diaphragm. With the proper compensation procedures to account for zero balance and thermal effects, these devices are very useful in the determination of fluid pressure levels in wind tunnels and other environments and also are sufficiently inexpensive to allow large numbers to be used in a research facility.
As with any other strain gauge device, there are limits to the range and accuracy of this specific strain gauge installation. For example, the accuracy of these devices depends upon the minute deflection of a thin diaphragm with a strain gauge installed on its surface. The application of pressure to this diaphragm causes it to deflect, thus introducing in the surface a strain which is detected by the strain gauge and which is also proportional to the load applied. Appropriate calibration and compensation procedures permit the applied pressure to be measured as a function of the electrical output of the strain gauge.
Range limitations occur because of the inherent restriction imposed by the applied pressure level to which the diaphragm can be exposed before exceeding either its linear response range or its ultimate stress limit, thus deforming the diaphragm and the strain gauge into a region of either nonrepetable results or failure.
In order to obtain maximum accuracy, it is desirable to maximize the amount of deflection. In other words, it is preferably to obtain a large deflection for a small applied pressure. However, in doing so, a transducer is limited in its amount of permissible overpressure, thus limiting its range of accurate measurement.
This conflict between desired accuracy and maximum range results in users being left with a choice between very accurate transducers with a limited range of safe overpressure and less accurate transducers that can be safely exposed to a wide range of high overpressure.
One method of minimizing this problematic choice involves electronic conditioning and amplifying of the output signal using a so-called auto ranging technique by which a variable gain is applied to the strain gauge output such that the final output simulates a nearly full scale reading, regardless of the applied pressure.
A major disadvantage of this method is that noise is also amplified along with the electrical signal. Furthermore, the output is still in reality the result of a small movement of the diaphragm.
Thus, the solution before undertaking most tests is to match carefully the transducer size with the pressure range expected to be encountered. Of course, this solution is not applicable to tests wherein the measured fluid pressure varies over a wide range.